The boat propelling system makes use of the impeller to compress water current to generate the backward thrust of water to propel the boat. As shown in FIG. 1, a boat propelling system of the prior art consists of an engine 12 housed in a hull 10, a shaft 13 driven by the engine 12, a water tract 11 housed in the hull 10, a propeller 14 mounted on the shaft 13, and a stator 15 located in the water tract 11 and composed of an axial portion 151, a plurality of stator blades 152, and a circular frame 153 holding the stator blades 152. In operation, the propeller 14 is driven by the shaft 13 to rotate to compress the water in the water tract 11. As water is forced to pass the stator 15, the water is guided by the stator blade 152 to flow in a linear direction before the water is discharged via a nozzle 111 to generate the backward thrust of water to propel the boat hull 10.
In order to enhance the backward thrust of water by the nozzle 111, the interior cross sectional area of the nozzle 111 is progressively reduced so as to accelerate the flow speed of water. According to the Bernoulli equation, an increase in the backward thrust of water can be attained by a gradual reduction in the interior cross-sectional area of the nozzle 111. However, such a nozzle 111 as described above is limited in design in that the water acceleration is often undermined by the inner wall of the nozzle 111 which is shortened, and that the hull 10 must be lengthened to accommodate the lengthened nozzle 111. In other words, an effective improvement in the boat propelling system can not be attained by redesigning the nozzle 111.